1. Statement of the Technical Field
The inventive arrangements relate generally to bandpass filters, and more particularly to a tunable bandpass filter and a method of tuning a bandpass filter.
2. Description of the Related Art
The designation “legacy” is often used to refer to manpack radios that were used by tactical forces during the decades which preceded and overlapped with the so called digital era. Overall performance of legacy radios was considered to be more than adequate, and is often used to judge the performance of similar communication devices used today. The good performance of legacy radios is in part due to the fact that they used waveforms with simple modulation schemes and narrow frequency coverage.
The comparatively simpler modulation schemes and limited frequency coverage of legacy radios lend themselves to transmit paths with very good transmit noise floor and transmit spurious performance in a small form factor with low power consumption. As an example of simple modulation schemes and narrow frequency coverage, some legacy radios may use FSK modulation with a frequency coverage of only 30-88 MHz.
In today's tactical environment however, manpack and handheld radios are required to operate in a congested frequency spectrum. Further, newer, multi-band, multi-mode SDR radios (software defined radios) require complex modulation schemes and broad frequency coverage. Modulation schemes such as QAM (quadrature amplitude modulation) require superior transmit linearity. Further, wider frequency coverage, for example, from 30-512 MHz, requires multiple bands of filtering. This requirement is at odds with requirements such as lower power consumption in a small form factor with high linearity.
It is a continuing challenge is to design a tunable bandpass filter to provide broadbanded and highly selective transmit filtering in a small form factor with minimal power consumption (legacy performance). Some “legacy” collocation specifications included transmit noise floor (−170 dBc/Hz), and transmit spurious (−80 dBc). These specifications (along with others) may be used to define “legacy performance”. To meet these specifications, selective filtering is required as close to the antenna as possible, i.e. selective filtering at the highest power level possible. However, legacy waveforms typically do not require high linearity transmit paths, that is, good IMD (intermodulation distortion) performance. This somewhat eases the linearity requirements of the filter. In contrast, newer high data rate/narrow bandwidth waveforms require a highly linear transmit path.
As one possible answer to the challenge, varactor tuned bandpass filters may be considered. However, varactor tuned bandpass filters have poor linearity, thus limiting their input power levels, and thereby reducing their filtering effectiveness from a transmit noise floor perspective. Linearity of the filter (intermodulation distortion) and tuning voltage are limited by the maximum voltage rating of the varactor diodes themselves.
Key factors for a tunable filter implemented with varactor diodes are the varactor tune voltage and the RF signal voltage. In a varactor tuned bandpass filter, as the RF signal voltage becomes significant compared to the varactor tune voltage, distortion increases dramatically. The varactor tune voltage is limited by the tuning capacitance change required (filter frequency coverage), and the maximum reverse voltage of the varactor diode (typically 25 VDC or less).
In one particular investigation, linearity measurements from prototypes of a varactor tuned bandpass filter were determined through standard measurements of intermodulation distortion. With −2.5 dBm PEP RF (peak envelope power radio frequency) signal input, the IMD products were 44.8 dB down. Given an insertion loss of −4.5 dB for the filter, the output intercept point (OIP) was +9.4 dBm.
As another possible approach to designing a software defined radio to meet legacy specifications, it has been suggested that linearity may be improved using a PIN diode switched bandpass filter. However, with a PIN diode switched bandpass filter, power consumption would be unacceptable in that the filter would require significant DC biasing circuitry and size. Without building a prototype, investigations suggested that a 20 dBm PEP RF signal input (100 mW) would have a +50 dBm output intercept point (OIP3). The power consumption of the filter would be approximately 350 mW given 10 mA PIN diode forward bias on 10 separate PIN diodes. This does not include the power required for diode switching circuitry. A PIN diode switched bandpass filter is impractical with respect to legacy specifications, at least because it does not meet the low power consumption requirement.
An objective of the invention is to provide a tunable bandpass filter having broadbanded waveforms with low distortion (high linearity) in the transmit path. A further objective of the invention is to provide a tunable bandpass filter having highly selective transmit filtering. Another objective of the invention is to provide a tunable bandpass filter having good collocation performance. A further objective of the invention is to provide a tunable bandpass filter having a small size. A further objective of the invention is to provide a tunable bandpass filter having low power consumption. A further object of the invention is to provide a software defined radio having legacy performance.